Aims: Reproductive problems (such as infertility, miscarriage, and birth defects) have been linked to environmental toxins in laboratory animals. However, little is known about similar effects in humans. Human reproduction is not only a likely target for environmental toxins, but it may be a more sensitive endpoint than cancer or other chronic disease, which can require many years to become apparent. Thus, reproductive problems may serve as an early signal of toxic exposures. Our goal is to describe the basic biology of human reproduction, to develop improved epidemiologic tools for detecting environmental damage to reproduction, and to identify specific environmental factors that interfere with human reproduction. Accomplishments: We previously found that women are not fertile after their day of ovulation. We have confirmed this finding in an independent analysis of another data set, after correcting for errors in the measure of ovulation. This confirmation casts new doubt on the widespread clinical assumption that women are able to conceive with intercourse as late as three days following their ovulation. In a further extension of our fertility research, we have shown that the natural (and unpredictable) variability of ovulation causes a given woman's fertile days to be extremely predictable. In particular, we found that even as late as the fourth or fifth week of the menstrual cycle, when women expect their next period to begin, a small but persistent fraction of women have not yet ovulated and are still capable of conceiving. This information is useful for women who are not using dependable methods of birth control and yet do not want to become pregnant. After the first few days of the cycle (during which most women are experiencing their menses), there is no day of the menstrual cycle on which women can be confident they are not fertile.